A vehicle equipped with the so-called Smart Key System (registered trademark) establishes bi-directional communication with a mobile device carried by a user to perform key authentication and unlocks a door under the condition that key authentication has been accomplished. For example, the vehicle transmits LF signals from a transmitter to form an LF area around the door in order to monitor the approach of the mobile device. When the user carrying the mobile device enters the LF area, the mobile device sends an RF signal in response to an LF signal. The vehicle analyzes the RF signal and permits unlocking of the door when key authentication is accomplished. In this case, the door actually is unlocked when the user touches the door handle.
A technique that incorporates an acceleration sensor in the mobile device has been developed to prevent unauthorized actions performed with a relay. When the acceleration sensor detects vibration of the mobile device, the reception of the LF signals is permitted. Further, when the acceleration sensor does not detect vibration of the mobile device, the reception of the LF signals is prohibited (for example, refer to patent documents 1 to 3).
A further developed technique incorporates an acceleration sensor in the mobile device to detect motions of the user and operate the vehicle when a predetermined motion has been detected or to use the motion of the user as an authentication condition (for example, refer to patent documents 4 and 5).